I see the problem. For the surface air temperature variable
data were available at all grid points. For SST there are some
grid points [ ~33% :-) ] that are all missing because they are
over land. The underlying fortran routine correctly set the internally
calculated lag-1 autocorrelation as missing (_FillValue). At this point
it should have returned with the equivalent sample size set to
_FillValue also. Unfortunaltely, it did not.

A fix will be applied in the near future.

Regards
Dennis Shea

>Hi. I had a question about equivalent sample size and ftest.
>I created variables sst1(lat,lon,time) and sst2(lat,lon,time),
>and applied a low-pass filter to them. Then I calculated their
>variances. For land pts. I use the FillValue of 1e+35.
>Now, I want to determine if their variances are significantly
>different using the ftest. I included my approach below.
>However, the equiv_sample_size command leads to this error:
>
> ==>SLATEC/NCL: DGAMMA:
> ANSWER LT HALF PRECISION BECAUSE X TOO NEAR NEGATIVE INTEGER: NERR=